The present invention relates to an overcurrent switching device.
Protective switches in the form of overcurrent switches have been known from the prior art for many years. They have the object of preventing a high current flow in an electric circuit caused for example by a short circuit by interrupting the electric circuit, as a result of which further dangers and problems such as for example damaging a consumer, accident risk or the like can be minimised.
It is also in particular known from the prior art, in addition to conventional technologies such as the use of bimetals, to also use so-called shape memory alloys (abbreviation: MSM, magnetic shape memories), namely those materials which show a change in length as a reaction to an applied magnetic field (typically an expansion of the material). This magnetic expansion effect is utilised for a multiplicity of applications, and for example has gained entry into electrical switching and safety technology for example on the basis of the teaching of DE 10 2004 056 280 A1. Furthermore, MSM alloys are generally also so-called thermal shape memory alloys at the same time. In addition to the structural change within the martensite which forms the basis of the MSM effect, there is namely also a phase conversion between martensite and austenite, which typically also leads to a length change of a corresponding body.
In the MSM technology mentioned and called upon to form the generic type, the current to be monitored for overcurrent flows through a coil which therefore becomes part of the electric circuit to be monitored and/or protected against overcurrent, and creates a current-strength dependent magnetic field there which acts upon an MSM material (which is provided for example in the manner of an armature in the coil in the prior art described). An exceeding of a current-strength threshold value predetermined by the expansion characteristics of the MSM element leads to the intended length change of the MSM element being effected and a switching contact provided (typically at the end) on the MSM element then interrupts the electric circuit in the manner of a protective switch functionality and thus effects the desired overcurrent protection.
A procedure of this type however initially has the disadvantage that substantial hardware or circuit outlay is necessary: In addition to the MSM element to be provided or fastened in a suitable manner, this must magnetically interact with the coil unit (which forms part of the electric circuit) and be suitably configured and set up, furthermore such a coil/MSM switching element combination is not arbitrarily universally usable, as for each use case (with a current threshold for electric circuit interruption to be monitored in each case) a respectively individual adaptation of a coil (for creating the necessary magnetic field) relative to the MSM element is necessary.
A further disadvantage in principle consists in the action of the coil as inductor, so that particularly in the case of a rapid sudden increase of the current, this is delayed (due to the inductance) and insofar induces a correspondingly slow triggering by means of the MSM element. In short-circuit situations or the like in particular, a procedure of this type is therefore sluggish on account of the system.
A device which additionally shows the features of an overcurrent switching device is known from WO 2007/057030 A1. For further prior art, reference is made to WO 2008/098531 A1 and also EP 1 610 418 A2.